The present invention relates to a process for washing and obtaining solids of a slurry. More particularly, it relates to a process for washing and obtaining the adduct of bisphenol A with phenol, said process being used for the production of high-purity bisphenol A.
Bisphenol A is used as a raw material for polycarbonate resins and epoxy resins. There is an increasing demand for colorless and high-purity bisphenol A (superior in quality to the conventional ones) which meets the requirements of polycarbonate resins for optical applications.
Bisphenol A is produced by reacting acetone with excess phenol in the presence of an acid catalyst and an optical co-catalyst such as a sulfur compound. The product mixture contains, in addition to bisphenol A, the catalyst, unreacted acetone, unreacted phenol, water, and by-products.
The by-products contain as major components 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (referred to as o,p'-isomer hereinafter) and Dianin's compound. Minor components include trisphenols, polyphenols, and coloring substances. They have adverse effects on the performance of the resins produced from such bisphenol A.
There have been proposed many processes for removing these impurities from such bisphenol A and obtaining high-purity bisphenol A.
One way of obtaining high-purity bisphenol A from the product mixture comprises removing the catalyst, unreacted acetone, water, and a small amount of phenol from the product mixture by vacuum distillation, cooling the residual liquid mixture, thereby causing bisphenol A to crystallize out in the form of an adduct with phenol, separating the resulting crystals from the mother liquor containing by-products, and removing phenol from the adduct, thereby obtaining bisphenol A.
According to another way, high-purity bisphenol A is obtained by recrystallizing from a solution of crude bisphenol A.
The removal of phenol from the adduct of bisphenol A with phenol may be accomplished by distillation, extraction, steam stripping, or the like. According to a process disclosed in U.S. Pat. No. 3,936,507 and Japanese Patent Publication No. 42790/1977, the adduct is vaporized for 0.1 to 30 minutes under reduced pressure at 180.degree. C. or above for separating and condensing bisphenol A. According to a process disclosed in Japanese Patent Publication No. 23335/1961, the adduct is heated to 50.degree. C. or above in a solvent having a boiling point higher than 50.degree. C. so that the phenol moiety alone is dissolved in the solvent.
The above-mentioned processes give bisphenol A that can be used as a raw material of general-purpose epoxy resins for example. However, this bisphenol A does not have a high purity satisfactory for use as a raw material of polycarbonate resins and other linear polymers. It needs further purification by recrystallizing from a hot aqueous solution (as disclosed in Japanese Patent Laid-open No. 62543/1984) or washing with a solvent (as disclosed in U.S. Pat. No. 4,492,807 and Japanese Patent Laid-open No. 231033/1984).
Heretofore, it has been believed that the above-mentioned post treatment is necessary to remove these impurities completely from bisphenol A because the adduct crystals themselves contain these impurities and hence they give bisphenol A of low purity after they are freed of phenol. However, we found that the adduct crystals of high purity do not necessarily give bisphenol A which has a satisfactory hue. Presumably, this is attributable to trace amounts of the mother liquor remaining on the adduct crystals separated from the mother liquor.
The adduct crystals may be freed of the mother liquor by washing them with phenol continuously or intermittently in the separator or by reslurrying them in fresh phenol after being discharged from the separator. The former method does not perform complete washing, and the latter method needs at least as much phenol as the adduct crystals for transferring the slurry to the subsequent separation step.
Examples of washing processes are disclosed in U.S. Pat. No. 3,049,568 and 3,221,061 and Japanese Patent Publication Nos. 981/1964 and 4454/1968. According to these processes, the centrifuged adduct crystals are suspended again in as much phenol as 70 wt% of the adduct crystals, and the resulting slurry is fed to the separator again and washed again with as much phenol as 20 wt% of the crystals.
This process has a disadvantage of decreasing the yield of the adduct crystals because bisphenol A is somewhat soluble in phenol.